Discovery of the eclogite and its petrography in the Northern Dabie Mountain
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Omphacite
Amphibole
Ultramafic rock
Peridotite
Hornblende
The Journal of the Japanese Association of Mineralogists Petrologists and Economic Geologists (1986)
It has been known that eclogites have a diversity in the mode of occurrence and petrology. In this paper, eclogites and eclogitic rocks are divided into two types, i.e., prograde and retrograde eclogites, based on the P-T paths of the equilibration leading to the eclogite facies mineralogy. The prograde eclogite was recrystallized from hydrous phases, and the retrograde eclogite is defined by its formation from higher temperature anhydrous phases. The following features help to identify the prograde eclogites: I. Regional distribution of eclogite facies a) Mapping of prograde mineral zones to reveal the dehydration reaction to form eclogites. b) Mapping of the progressive thermal gradient in the zone of the eclogite facies. II. Presence of such hydrous minerals as epidote, amphibole, chlorite and micas as the inclusions in the eclogitic garnet and omphacite. III. Progressive zonal structure of the eclogitic garnet and omphacite. Based on these criteria, the eclogites from New Caledonia, Tauern Window and Sesia Lanzo zone in the Alps, Caledonian belt in Norway, and Sebadani in the Sambagawa metamorphic belt are classified into the prograde eclogite. At least two metamorphic facies, the glaucophane schist and epidote amphibolite facies, are confirmed for the pre-eclogite stage of the prograde eclogite. The transition from the glaucophane schist facies to the eclogite facies has been described in the New Caledonia and Sesia Lanzo zone, while that from the epidote amphibolite facies to the eclogite facies is observed in the Tauern Window, Norway and the Sambagawa metamorphic belt. The nature of the metamorphic facies on the lower grade side of the eclogite facies depends on the pressure, and the higher pressure prefers the glaucophane schist to the epidote amphibolite facies.
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Abstract A largely undocumented region of eclogite associated with a thick blueschist unit occurs in the Kotsu area of the Sanbagawa belt. The composition of coexisting garnet and omphacite suggests that the Kotsu eclogite formed at peak temperatures of around 600 °C synchronous with a penetrative deformation (D1). There are local significant differences in oxygen fugacity of the eclogite reflected in mineral chemistries. The peak pressure is constrained to lie between 14 and 25 kbar by microstructural evidence for the stability of paragonite throughout the history recorded by the eclogite, and the composition of omphacite in associated eclogite facies pelitic schist. Application of garnet‐phengite‐omphacite geobarometry gives metamorphic pressures around 20 kbar. Retrograde metamorphism associated with penetrative deformation (D2) is in the greenschist facies. The composition of syn‐D2 amphibole in hematite‐bearing basic schist and the nature of the calcium carbonate phase suggest that the retrograde P–T path was not associated with a significant increase or decrease in the ratio of P–T conditions following the peak of metamorphism. This P–T path contrasts with the open clockwise path derived from eclogite of the Besshi area. The development of distinct P–T paths in different parts of the Sanbagawa belt shows the shape of the P–T path is not primarily controlled by tectonic setting, but by internal factors such as geometry of metamorphic units and exhumation rates.
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Kyanite-bearing eclogitic assemblages occur in the highest-grade zone of the Sanbagawa metamorphic belt, central Shikoku, Japan. The eclogites consist mainly of garnet, omphacite, phengite, kyanite, epidote, quartz and rutile. Compositionally variable amphibole (glaucophane/barroisite/pargasite), phengite and paragonite occur as inclusions in garnet and other eclogite facies phases. Careful examination of garnet zoning in kyanite-eclogites suggests that (i) most garnet grains show complex zoning consisting of relatively Ca-rich/Mg-poor inner and Ca-poor/Mg-rich outer segments, (ii) the inner segment of the zoned garnet formed at the eclogite facies stage, and (iii) the Mg-rich outermost rim of garnet does not always represents a composition at peak eclogite stage but could form at lower-pressure conditions of subsequent epidote-amphibolite facies. The assemblage of inner segment of garnet, omphacite, phengite, kyanite and quartz points to equilibrium conditions of 2.3-2.4 GPa/675-740 °C. The metamorphic P-T conditions of the eclogite facies stage reported in literature have been estimated assuming that the outermost rim of garnet with Mg-rich composition was in equilibrium with other eclogite facies phases. Therefore, P-T estimations of the eclogite facies stage in the Sanbagawa metamorphic belt should be re-examined carefully on the basis of textural and compositional heterogeneities of constituent minerals.
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